Glossary of "Ecology (6th ed.)" by Krebs (2009)
- in a broad sense, the living together of two or more organisms of different species; in a narrow sense, synonymous with mutualism.
Glossary of "Lichens" by Purvis (2000)
- The living together of unlike organisms; it may be for mutual benefit or not.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- The living together of two or more species in a prolonged and intimate relationship.
Glossary of "Environmental Science: Earth as a Living Planet (2nd ed.)" by Botkin & Keller (1998)
- An interaction between individuals of two different species that benefits both. For example, lichens contain an alga and a fungus that require each other to persist. Sometimes this term is used broadly, so that domestic corn and people could be said to have a symbiotic relationship—domestic corn cannot reproduce without the aid of people, and some people survive because they have corn to eat.
- Botkin, D. B. & Keller, E. A. 1998. Environmental Science: Earth as a Living Planet, 2nd ed. -xxxxii + 649 pp. John Wiley & Sons, Inc., New York.
- Krebs, C. J. 2009. Ecology, 6th ed. -xvi + 655 pp. Pearson Benjamin Cummings, San Francisco.
- Purvis, W. 2000. Lichens. 112 pp. Smithsonian Institution Press, Washington.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.